Typically, the specific modulation of an immune response to an antigen in a subject requires the administration of another substance, e.g. an adjuvant, in admixture with the antigen in order to initiate and/or direct the modulation. Traditional adjuvants, though, have a number of weaknesses. For example, many are crude, heterogeneous preparations. In addition, many are relatively weak immunomodulators, and some cause severe local inflammation that is unacceptable in humans. Purified soluble polypeptides, such as cytokines, have some advantages over crude adjuvants, but their value is limited because they diffuse away from the antigen upon administration. While certain cell-surface molecules may be potential immunomodulators as components of cell-based vaccines, their application generally involves gene transfer into the cells, which is often problematic.
The invention therefore fills heretofore unmet needs by providing molecules that can bind to antigen bearing targets, such as cells, viruses, and isolated antigens, and that can serve as immunomodulators when administered with an antigen bearing target. In addition, the invention provides compositions comprising these molecules and related methods. The compositions and methods of the invention are also useful for other applications, e.g. any application in which it is desirable to attach a biological effector, such as a polypeptide ligand for a cell surface receptor, to a target structure, such as a virus or a cell.